Fixed comment on maximum tested size for POLARSSL_MPI_MAX_LIMBS
Correctly handle CertificateRequest with empty DN list in <= TLS 1.1
Added ssl_handshake_step() to allow single stepping the handshake
process

Single stepping the handshake process allows for better support of
non-blocking network stacks and for getting information from specific
handshake messages if wanted.
Added PolarSSL 1.1.5 ChangeLog from 1.1 branch
Updated for PolarSSL 1.2.4